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Try heating the mild up to casting temp. Take a stick of bullet lube and apply a small amount to the stuck on lead. Then run molten lead over the deposit, build a pile of lead up over the open cavity then pry it off with your pop cycle stick. You may need to try this a few times to get the stuck on lead off. Use a paper towel to wipe the loose spots off.

The beeswax did the trick. I heated it up on the hot plate, smeared a bit of beeswax over the offending area and let it sit just a little and took a small dowel rod and then tried a toothpick and it came off. That cavity is discolored now but as long as it still cast well, no matter. This is the first brass mould I've used the 1:20 MIX with. The other brass moulds are pistol moulds and with those I use ww's plus a couple ounces of tin or I'll mix an equivalent 50/50 lead to Linotype and add lead to give me a BHN11-12 or close. Never had this problem with those moulds. Obviously, the tin is the problem. Wish I had known this early on I would ordered an iron mould. Now, to keep this from happening again. In the future any mold I order for large cavity bullets destined to use 1:20 or 1:30 will be iron or aluminum.

That photo in post #12 is the worse I've seen. I'm sure glad this one wasn't that bad.

Springfield is right, it isn't lead, or tin, that's why it won't melt away. It is accumulated oxides of mostly tin, it starts a 'seed' in a shiny spot and just builds and builds a few molecules at a time from the flash-oxidized molten surface of the metal you pour into the cavities. It's the same sort of stubborn crud that clings to the sides of a casting furnace as the alloy level drops. A reducant like beeswax or paraffin quickly takes care of it once you get the mould up to the melt point of the elemental metal.

Being an oxide, and beeswax being a reducant as JonBinGlencoe pointed out above, you can heat the mould to the point that the tin would start to melt if it weren't oxidized, apply the wax to instantly reduce the oxides to elemental, liquid metal, and wipe them away.

It happens to me with brass moulds because they do like to be run hot, it IS worse with binary lead/tin alloys, and the brass moulds DO like to be run hotter/faster than other types which DOES exacerbate the oxide accumulation.

What I do to patina a brass mould for the first session is preheat it to casting temp in a mould oven and let it cool naturally a few times, then smoke the cavities lightly with a BBQ grill lighter before ever putting lead in there. That keeps those little soldered "seeds" from ever starting. After a few casting sessions, I wipe out the soot and leave them alone. This is the only time I ever smoke moulds because it can cause a lot of other problems, but it really helps break-in with brass.

Gear

Excellent Post! I had not considered oxides. It certainly explains why the deposits are so tough and why beeswax works.

Curious though, what is the trigger, why that spot? The underside of the groove (as cast with base up) at the block face seems to be The Spot.

Since I had the mould cleaned up from all that tin or oxides, whichever, I decided to cast some with it today. Had to pound on the handle pivot again but this time I noticed the deposits on the right half of the mould instead of the other side. Aggravated, I hit it with the beeswax stick and let it cook in and then took a wooden paint stick to it and they flicked right off. Went back to casting with little trouble after that other than the half dozen that were wrinkled due to the wax. Maybe sooner or later it'll stop this.

All my brass moulds have worked fine, but then my Mantra is "tin is over-rated, use more heat!". A good casting cadence can solve many problems. That plus I don't clean my moulds all that much when new, especially the better quality NOE, BRP, Accurate and Mihec. I have on occasion done nothing to a Mihec mould except heat it up a couple times to burn off the oil, then cast. My moulds don't look nice and shiny but they are tools, not art, so I do what works the best. Lee moulds I do a bit of scraping off of chips and facing off the front of the sprue plate, and stake the pins in place, but not much clean-up except a quick bath with dish soap.

I seldom add any tin and then it is 50/50 solder bar. Usually, I simply flux regularly and cast fast and fairly hot to get fillout using mostly range scrap or wheelwheights. No tinning (or tin oxide or whatever) problem for me.

My boolits are usually slightly frosty and that suits me.

After some use a natural patina should form and protect the mould but an artificially produced patina could be the answer as Mal Paso was working on.

Which ones, large cores, small containers or other. The large cores are 3% antimony and 1% tin.

I don't know what they originally looked like as I got them in ingots some time back during a group buy. Last time I checked them with the Lee they were BHN10. I can't recall who had them for sale on the forum. It just came to me...Muddy Creek Sam, last June a year ago during a group buy.